DC ground fault detection

ABSTRACT

Apparatus for the detection of ground leakage in a normally ungrounded DC system includes a DC power supply and conductors from the supply for supplying power to a load connected to the DC system. There is an indicator connected between the DC system and a ground point such that a ground leakage in the system closes a circuit to activate the indicator. With such activation, a pulser periodically interrupts the circuit to effectively generate an interrupted ground fault signal. Such signal is detected by a sensor located relative to the DC system such that an interrupted ground signal detected by the sensor locates the ground fault. Noise introduced into the pulse signal by the periodic interruption of the circuit during the ground fault occurrence is suppressed by the sensor.

This is a continuation-in-part of application Ser. No. 773,333, filedSept. 6, 1985. The contents thereof we incorporated by reference herein.

BACKGROUND OF THE INVENTION

This invention relates to the detection of DC ground faults. Inparticular it relates to an apparatus and a method for detecting suchfaults in normally ungrounded DC distribution systems having significantcapacitive reactance components and strong electromagnetic andelectrostatic fields associated with utility power generation anddistribution, industrial plants, and computer/electronic systems, whereground faults must be located without taking unaffected equipment out ofservice.

A basic problem in such systems is the need to identify small DC faultcurrents namely low to high impedance ground faults in the presence ofmuch larger DC load currents.

One well-known ground detection circuit consists of a center tapped highresistance connected across the DC source and an indicating milliammeterbetween the center tap and ground. A ground fault anywhere on the DCsystem causes an indication on the milliammeter. Since the highresistance limits the ground fault current to a few milliamperes thecircuit is not tripped off when a fault occurs. This is important sinceloss of power on a typical DC control circuit is often critical andcould involve safety hazards. It is important to locate and repair anyground faults as soon as practical since a second ground fault wouldtrip the circuit.

Such a ground detection system while indicating that a ground fault hasoccurred does not tell where the fault lies. It could be in any one ofmany pieces of equipment on numerous branch circuits. Again because ofthe critical nature of these circuits it is not practical to turn themoff one at a time to locate the fault. Thus a system is needed to locatethe faulty equipment without interrupting these critical circuits.

Another system for DC fault detection requires the introduction into aDC fault line of an AC current at a frequency of about 25 Hz, which isthen detected. A problem with the introduction of such AC current isthat it is liable to cause operational problems, mask some faults, andcreate complications in detecting and localizing ground faults in someDC loads in the system.

It is also known elsewhere to test for DC faults in small systemsemploying grounded 12-volt battery type power supplies in automobilesand the like. Such grounded DC systems require the connection of aninjector across terminals of the battery supply and thereafter adetector is applied over the wiring system with sound detection meanssuch that an increasing sound would indicate where a DC fault exists.

Such systems operate in response to high DC fault currents in anenvironment where there is no capacitive or inductive reactances ofconsequence, or where they are of no real concern and where the DCsystem is effectively shut off when the fault detection is being made.

It is also known in AC systems to detect ground leakage by a relay whichinterrupts the system so as to introduce a fault current in the sense ofa pulsating input. Such systems however are of a nature that aD'Arsonval type meter or permanent magnet moving coil meter are used fordetection of the pulsating input. Such a meter requires a currenttransformer suitable for detecting relatively large AC fault currents,and this is unsuitable for measuring pulsating DC fault currents of alower value. These detection systems are particularly unsuitable in highelectrostatic and electromagnetic environments.

SUMMARY OF THE INVENTION

According to the invention there is provided apparatus for the detectionof ground leakage in a normally ungrounded DC distribution system whichincludes a DC power supply and conductors from the supply for supplyingpower to load means connected to the DC distribution system. There is atapping point between resistor means connected across the DC powersupply and responsive means, such as indicator means, connected betweenthe tapping point and a ground point, such that a ground leakage in thesystem completes a circuit to activate the responsive means. With suchground leakage activation, there are means adapted to interruptperiodically the circuit to effectively generate an interrupted groundfault signal. Such signal is detected by either a permanently locatedand/or portable sensor or means located relative to the DC system suchthat a pulse interrupted ground signal can be detected by the sensormeans and thereby the ground fault located in the DC system.

The sensor means includes means to suppress noise introduced into thepulse signal by the periodic interruption of the circuit during theground fault occurrence, and also includes means to eliminate theeffects of distributed capacitive and inductive reactance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating a DC system with various loads,and in which a ground fault is present in one of the loads, includingsensor means for sensing such fault.

FIG. 2 is a block diagram schematic of the sensor means for detectinginterrupted ground fault signals.

FIG. 3 is a detailed schematic of the sensor means of FIG. 2.

FIG. 4 is second block diagram schematic of the sensor means fordetecting interrupted ground fault signals.

FIGS. 5A and 5B are detailed schematics of the sensor means of FIG. 4.

DETAILED DESCRIPTION OF THE DRAWINGS

Apparatus for the detection of ground leakage in a normally ungroundedDC system (FIG. 1) comprises a DC power battery supply 10. Main bus bars11 and 12 from the battery supply 10 supply power to different loads 13,14 and 15 in this exemplary embodiment. Conductors 13a and 13b from mainbus bars 11 and 12 connect with load 13. Similarly, the main bus bars 11and 12 are connected to load 14 through conductors 14a and 14b. Thereare conductors 15a and 15b to load 15.

Across the bus bars or conductors 11 and 12 are resistors 16 and 17 andbetween these resistors 16 and 17 is a tapping point 18. A responsiveelement in the form of ground indicator meter 19 is connected betweenthe tapping point 18 and a ground point 20 such that a ground faultleakage in the system closes a ground circuit to activate the indicatormeter 19. In the conductor 21 connecting the indicator means 19 to aground point 20 there is located a reed relay 122 operable by a pulser22 to open and close the relay 122 at approximately one cycle every 6seconds. In this fashion a DC pulsing fault current is generated intothe ground circuit and thereby an interrupted ground fault signal isobtained.

The remainder of the ground circuit is constituted through the bus bars11 and 12 and conductors 14a and 14b to load 14 which is indicated tohave a ground fault 23. In the exemplary embodiment loads 13 and 15 donot have such ground fault.

The relay 122 is normally closed. The pulser 22 is incorporated in acircuit with a normally open switch 222 for selectively activating saidpulser 22 to operate said relay 122. Closure of switch 222 can bemanually effected or be by electromechanical means on the occurrence ofa ground fault condition.

In the one example of the invention, for each load circuit 13, 14 and 15there is provided a sensor 24 respectively. Such sensor 24 includes aHall effect sensing element 25 together with detection circuit 125 toindicate whether a pulsating ground signal is sensed by the Hall effectelement 24 in a particular conducting line 13a, 13b, 14a, 14b, 15a or15b respectively.

The pulser 22 is not placed into circuit until such time as the groundfault indicator 19 detects the existence of a ground fault current inthe embodiment described. In some cases, however, the indicator 19 maybe dispensed with, or the pulser 22 continually applied irrespective ofthe indicator 19 such that any permanently located sensor 24 willindicate a fault current.

The embodiment of FIGS. 2 and 3 is now described.

The sensor 24 for detecting interrupted ground fault signals asillustrated in block diagram of FIG. 2, includes a Hall effect sensingelement 25 which receives both a composite pulse signal 200 withsuperimposed noise 201 of which is fed from the Hall effect sensor 25along conductor 26. The signal is received from the pulser 22 in theform of a one-sixth (1/6) to one-eleventh (1/11) Hertz square wave 200.Accordingly the period is in the range of 6 to 11 seconds.

The power supply to the sensor 24 includes a battery source 30 which isnormally a small DC volt supply. This supply 30 is passed through afirst voltage regulator 31 which detects variations of the batterysupply, for instance, a decrease of the battery voltage over a timeperiod. The battery voltage sample is passed along line 32 to a secondvoltage regulator 33.

The first voltage regulator 31 stabilizes the battery voltage supply tothe overall circuit and makes it possible for the circuit to operatefrom 6.5 volts to 15 volts. The second voltage regulator 33 delivers avery stable voltage supply to a DC amplifier and low pass filter 34.Combined with the voltage regulator 31 the voltage regulation is within±0.02% and this eliminates the spurious injection of transients to theDC amplifier 34.

The DC amplifier and low pass filter 34 amplify the signal transmittedalong conductor 26 from the Hall effect sensor 25 and passes only thosefrequencies that are 5 Hertz or less. For proper operation, the outputvoltage range of the DC amplifier is within 0.75 volts to 2.75 volts.

Associated with the Hall effect sensor 25 is an input offset adjustmentelement 35 which permits for the manual adjustment to bring the outputlevel of the Hall effect sensor 25 to a value suitable for operation ofthe DC amplifier 34.

In order to ensure that the DC amplifier output voltage is in theappropriate range there is provided a center biased detector 36 withvisual indicator means 37 and 38 respectively indicating whether the DCamplifier is set to operate in its proper range. With the visualindicator 37, namely, the hi-LED lit-up there is indicated that the DCamplifier output is greater than 2.75 volts. With the indicator 38,namely, the "low-LED" lit-up there is indicated that the DC amplifier 34output is less than 0.75 volts. Thus, when both the indicators 37 and 38are in the "off" state the DC amplifier 34 is correctly set up.

Also located between the voltage regulator 31 and Hall effect sensingelement 25 is a temperature compensating current regulator 39 whichprovides a constant 20 mA current to the Hall effect sensor 25 forcontrol of the current requirements.

The output from the DC amplifier and low pass filter 34 is passed alongconductor 40 to a slope detector 41. The output from the detector alongline 42 changes from a high to low or low to high state when the outputwave form 43 from the DC amplifier low pass filter 34 changes at a rateof 5 mV per second or more from the point of inflection of the wave formcurve.

The pulse type square wave output 44 from the slope detector 41 is fedby conductor 42 to a pulse edge detector 45. The detector 45 outputsalong line 47 a positive spike 46 for every rise or fall edge of theslope detector output wave form 44. The output spike wave form 46 whichis fed along conductor 47 to a one shot multivibrator 48 which outputs a0.6 second duration pulse, for every pulse from the pulse edge detector45. The one-shot multivibrator 48 the outputs this pulse 49 along theoutput conductor 50 which feeds a driver amplifier 51. The amplifier 51operates either an LED indicator 52 or buzzer 53 to provide a visual oraudio means of recognizing the existence of a fault.

The voltage regulator 31 is also connected along line 54 with the driveramplifier 51, the one-shot multivibrator 48 and the pulse edge detector45. Alternatively, to a LED 52 or buzzer 53 there can be a meter 55operated by the driver amplifier 51 as indicated in FIG. 1.

FIG. 3 discloses in more detail the circuitry constituting the sensorillustrated in the block diagram of FIG. 2. The Hall sensor device 25 isshown connected through conductor 26 via an input offset adjustconstituted by a variable resistor bank 55 to the IC elements 56 and 57the output of which is in turn connected with the IC 58 to constitute anamplifier and a low pass filter.

The voltage regulator 33 is connected to the amplifiers by line 59 andto the center bias detector 36 along line 60 and to the slope detectorby line 61.

The voltage regulator 33 consists of several transistor configurations62, 63, and 64 arranged in voltage regulating fashion.

The voltage regulator 31 includes a capacitor 64, the positive side ofwhich is connected through line 65 with the temperature compensatingcurrent regulator which includes an array of transistor 66 and 67 anddiode 68 suitably arranged.

The output of the current regulator 39 is connected by conductors 69 and70 to the Hall effect sensing device 25.

Between the battery 30 and the voltage regulator 31 is a manuallyoperable switch 71 to activate the sensor 24 as desired.

The center bias detector 36 includes a pair of IC elements 72 and 73respectively, the outputs of which are connected to transistors 74 and75 respectively to in turn drive the LED's 37 and 38.

The slope detector 41 receives a signal from the DC amplifier and lowpass filter along the conductors 40 which is then passed through a firstIC element 76 and the output in turn to an IC element 77. The outputfrom IC 77 passes along line 78 to the pulse edge detector 45 andmultivibrator 48 along line 78. The pulse edge detector andmultivibrator are constituted by the array of transistors 79, 80, and 81suitable arranged. The output from the multivibrator transistor 81passes along line 50 to the driver amplifier circuit constituted by atransistor 82 which itself is arranged to drive either the buzzer 53 orLED 52 as necessary.

The embodiments of FIGS. 4, 5A and 5B are now discussed.

The sensor 324 for detecting interrupted ground fault signals asillustrated in block diagram of FIG. 4, includes a Hall effect sensingelement 325 which receives both a composite pulse signal 300 withsuperimposed noise 301 of which is fed from the Hall effect sensingelement 325 along conductor 326.

The output from the DC amplifier and low pass filter 339 is passed alongconductor 340 to an instrument amplifier and low pass filter 341, device341 transforms the differential input signal to an imbalance outputsignal.

The square wave output pulse 303 from the instrument amplifier 341 isfed by conductor 342 to operational amplifier and low pass filter 343.The output of operational amplifier 343 along line 344 is a square wavesignal. The output waveform 304 is fed along conductor 344 to amplifiercircuit 345, 346 and switch relay 350. The amplifier circuit 345operates the positive LED and negative LED to provide a visualindication of what line has a ground fault current. The output signal304 along conductor 344 is fed to amplifier 346, who controls the greenLED indicator. The output signal 304 along conductor 344 is fed to therelay switch 350; the input of relay 350 is controlled by line 362 whoreceived a synchronization signal 361 from interruptor circuit 322; theoutput signal of relay switch 350 is fed along conductor 351 to trackand hold circuit 352; the output signal of track and hold circuit 352 isfed into lines 353 to operational amplifier inverting circuit 354; theoutput signal of circuit 354 is fed to line 355 and to the input of 356.Amplifier circuit 356 receives a signal from Hall effect generator 325via conductor 326, amplifier circuit 356 has an adjust element whocontrols the DC current to be applied to the Hall effect generatordriver circuit 357. The H.E.G. DC driver circuit receives the compositesignal from circuit 356 and the control signal is applied to H.E.G. 325by means of conductor 359.

The signal is received from the pulser 322 in the form of a one-eleventh(1/11) hertz square wave 300.

The power supply (358) to, is a +5, -5 volts DC regulated power source,this power supply has an over-voltage, over-current protection circuitto supply power to the sensor and amplifier circuits.

The DC amplifier and low pass filter 329 amplifier the signaltransmitted along conductor 326 from the Hall affect sensor 325 andpasses only those signals that are 10 Hz or less. For proper operation,the output voltage range of the DC amplifier shall be within and +(plus) or - (minus) 200 milivolts in order to ensure that the DCamplifier output voltage is in the appropriate range. An adjustmentelement, 336, is provided with visual indicators, 347, 348, and 360,respectively, indicating whether the DC amplifier is set to operate inits proper range. With the visual indicator 347, namely the positive LEDilluminated, indicates that the DC amplifier output is greater, + (plus)by 200 microvolts. With the visual indicator 348, namely, the negativeLED illuminated, this indicates that the DC output of amplifier 339 isless - (minus) by 200 microvolts, thus, when both indicators 347 and 348are in the OFF position and indicator 360 green LED is illuminated, theDC amplifier 339 is correctly aligned.

Associated with the Hall generator is an adjustment element 356 whichpermits the manual adjustment of a constant current to bring the outputlevel of the Hall effect sensor 325 to a value suitable for operation ofDC amplifier 339.

FIGS. 5A and 5B disclose in more detail the circuitry constituting thesensor illustrated in the block diagram of FIG. 4. The whole sensordevice is shown connected through conductor 326 to differentialamplifier U4; the differential output of U4 is connected throughconductor 340 to the differential input of U1; the unbalance outputsignal of U1 is connected via conductor 342 to the operational amplifierand low pass filter S6; the output of S6 is applied via conductor 344 toamplifier integrated circuit S8 and S9; S8 will control the red positiveand red negative LED 347 and 348; S9 will control the green LED 360.Conductor 344 feeds switch relay 350 which opens and closes with thesynchronization signal 361 from the interruptor circuit 322 viaconductor 362; the output signal of switch 350 is fed via conductor 351to track and hold circuit 352; IC element S5, the output from IC S5passes along line 353 to the inverted circuit 354; this inverted circuitconsist of IC S4 and associate resistor and capacitors. The automaticgain control circuit 356 receives the signal from inverter circuit 354via conductor 355 which will feed back signal from the Hall effectgenerator 325 via conductor 326, this composite signal is fed to IC S3via conductor 327 to transistor Q1 via conductor 359 which controls thecurrent applied to Hall effect generator 325.

In operation of the DC fault detector the procedure is that the fault isfirst verified as existing in the system by observing the indicator 19located between the tapping point 18 and ground 20 or permanent alarmsystem or differential volt meter. This would indicate that a faultexists and should this be sufficiently large then a resistive faultcurrent would be indicated. The pulser 22 is then turned on by closingthe switch 222.

The Hall clamp-on sensing devices 24 as currently available aresensitive to a fault current of at least 3 milliamps. More sensitivedevices, however, could be available.

The sensing elements 25 are clamped over the conductors 13A, 13B, 14A,14B, 15A and 15B respectively, optionally, after verifying with thepercent ground meter 19 that the fault current is sufficiently largeenough to detect. Thereupon the input offset adjust 35, and in theembodiment of FIGS. 2 and 3, center bias detector 36 are adjusted so asto effectively render the sensor 24 operational.

The isolation of the detector fault current to the branch circuit 13A,13B, or 14A, 14B, or 15A, 15B is determined by a response to the pulsedinput signal by either the LED 52, buzzer 53, meter 55, or LED 47 or 48which constitutes the indicator means of the sensor 24 in the respectivebranch having a ground fault.

In the example illustrated the response will be in the branch line 14A,14B in view of the ground fault 23. The sensor 24 will in that circuitpass interrupted ground fault current as generated by the pulser 22through the reed relay 122 which is opened and closed in the groundcircuit. Accordingly, the indicator in the sensor 24 will respond. Inthose circuits where there is no ground fault there will be no indicatorresponse or an irregular response in the sensor 24. In the embodimentsof FIGS. 4, 5A and 5B there will be a green LED 60 response in thesensor 24. The indicator response in sensor 24 for a resistor faultoutput would be regularly indicated at about 3 second intervals. In theembodiment of FIGS. 4, 5A and 5B there will be a red LED 47 and 48response.

In some cases, by moving the sensor 24 along the conductors 14A and 14Bto a point where the ground fault signal ceases to be detected by thesensor it has provided a means for detecting the actual location of theground fault. The detector in fact need be placed only about eitherconductor 14A or 14B to locate more precisely the location of the fault.

Embodiments of the invention sensors can be permanently located atdiscrete points. Moreover, a pulser can also permanently be in circuitsuch that on the occurrence of a ground fault one or more sensors willrespond enabling the location of the ground fault. Essentially, theapparatus and method of the invention ensure that the normallyungrounded DC system can remain operational in respect of the ungroundedloads and this prevents expensive and unnecessary down time for systemswhich must continue operation while others suffer ground fault problemsand during detection of those problems.

Many changes and variations may be made in the apparatus and methodproviding widely different embodiments in applications for thisinvention without departing from the scope thereof. All matter containedin the above description as shown in the accompanying drawings shall beinterpreted as illustrative but not limiting, the invention beinginterpreted solely by the scope of the appended claims.

I claim:
 1. Apparatus for the detection of low level ground leakagecurrent in a normally ungrounded DC system in a multifeeder distributionsystem having a relatively high capacitive reactance and beingassociated with a relatively strong electromagnetic and electrostaticfield, said system including a DC power supply, conductors from thesupply for supplying power to a load connected in the DC systemcomprising a resistor element for connection across the DC power supply,a tapping point to the resistor element, a connector between the tappingpoint and a ground point such that a ground leakage current in thesystem completes a ground circuit, a normally closed relay between thetapping point and ground, a pulser for periodically opening and closingsaid relay while the ungrounded DC system remains substantially closedand operational, the opening and closing of said relay interrupting saidground circuit effectively thereby to generate a DC pulse interruptedground fault signal and a magnetic sensor for location relative to theDC system for sensing said magnetic field changes and thereby said DCpulse interrupted ground fault signal and thereby providing fordetecting the location of the low level ground leakage current in therelatively high capacitive reactance system, said DC system remainingsubstantially closed and operational during the ground leakage currentdetection, the pulse interruption rate being between about 6 to about 11cycles per second.
 2. Apparatus as claimed in claim 1, wherein saidsensor includes at least one of a meter indicator, LED display or soundindicator and said sensor indicates a presence of said pulse interruptedground fault signal by said meter indicator, LED display or soundindicator.
 3. Apparatus as claimed in either of claims 1 or 2, includingelectronic signal conditioning means for reducing noise generated by thepulser and relay.
 4. Apparatus as claimed in claim 3, including a signalpick-up element, and wherein the electronic signal conditioning meansincludes a DC amplifier and a low pass filter and means for biasing theoutput of the DC amplifier between a selected voltage range. 5.Apparatus as claimed in claim 4, including a DC battery source for thesensor, and means for minimizing DC battery voltage variation effects onsaid sensor.
 6. Apparatus as claimed in claim 5 wherein the means forminimizing DC battery voltage variation includes a first voltageregulator to detect variations in battery supply voltage, and a secondvoltage regulator to minimize transients.
 7. Apparatus as claimed inclaim 5, wherein the pick up element includes a Hall effect sensingelement for physical relationship about at least one conductor in the DCsystem.
 8. Apparatus claimed in claim 6 including a switch in circuitwith said pulser, said switch being normally open and being operative toclosure thereby to activate the pulser.
 9. Apparatus as claimed in claim5 wherein the pulser signal is substantially a square wave with a periodin a range of substantially 6 to 11 seconds.
 10. Apparatus for thedetection of low level ground leakage current in a normally unground DCsystem in a distribution system having a relatively high capacitivereactance, including a DC power supply, conductors from the supply forsupplying power to a load connected in the DC system comprising aresistor element for connection across the DC power supply, a tappingpoint to the resistor element, a responsive element connected betweenthe tapping point and a ground point such that a ground leakage currentin the system completes a ground circuit to activate the responsiveelement, a relay between the responsive element and ground pulser forperiodically opening and closing said relay, while the ungrounded DCsystem remains operational, the opening and closing of said relayinterrupting said ground circuit effectively to generate a DC pulseinterrupted ground fault signal to produce proportional periodic changesin a magnetic field associated with the system, said pulser beingincorporated in a circuit with a normally open switch, whereby on theoccurrence of a ground fault condition, said pulser operates said relay,and a magnetic sensor for location relative the DC system for sensingsaid magnetic field changes and thereby said DC pulse interrupted groundfault signal whereby solely said interrupted ground fault signalprovides the necessary signal for the low level ground fault current inthe relatively high capacitive reactance system detection, and includingelectronic signal and conditioning means for reducing noise generated bythe pulser and relay wherein said conditioning means includes anamplifier and filter for the noise, the pulse interruption rate beingbetween about 6 to about 11 cycles per second.
 11. Apparatus as claimedin claim 10, wherein the responsive element is a ground indicator meter.12. A method of detecting low level ground leakage current in a normallyungrounded DC system in a multifeeder distribution system having arelatively high capacitive reactance and being associated with arelatively strong electromagnetic and electrostatic field, said systemincluding a DC power supply, conductors from the supply for supplyingpower to a load connected in the DC system comprising the steps ofconnecting a resistor across the DC power supply, connecting anindicator between a tapping point relative to the resistor and a groundpoint such that a ground leakage current in the system closes a groundcircuit to activate the indicator means, periodically interrupting saidcircuit by opening and closing the ground circuit effectively togenerate a DC pulse interrupted ground fault signal to produceproportional periodic changes in a magnetic field associated with thesystem while the ungrounded DC system remains substantially operational,and magnetically sensing in a location relative the DC system saidmagnetic field changes related to said pulse interrupted ground faultsignal whereby said magnetic field changes provide the necessary signalfor low level ground fault current detection, the pulse interruptionrate being between about 6 to about 11 cycles per second.
 13. A methodas claimed in claim 12, including closing a switch to a pulser wherebythe pulser effectively signals the opening and closing of a relaythereby to generate said pulse interrupted ground fault signal.
 14. Amethod as claimed in claim 13, including filtering from the sensor noisegenerated by the signal interruption.
 15. Apparatus as claimed in claim3 wherein the indicator response in the sensor is indicated at about 6second intervals.
 16. Apparatus as claimed in claim 3 wherein themagnetic sensor is responsive to a leakage current of about 4 milliamps.17. Apparatus as claimed in claim 3 wherein the magnetic sensor isresponsive to a leakage current of less than about 4 milliamps. 18.Apparatus as claimed in claim 10 wherein the DC pulse interrupted groundfault signal has a period of at least about 6 seconds and wherein themagnetic sensor is responsive to a leakage current of less than about 4milliamps.
 19. A method as claimed in claim 12 wherein the sensorresponds to magnetic field changes at intervals of about 6 seconds. 20.A method as claimed in claim 12 wherein the magnetic sensor isresponsive to a leakage current of at least less than about 4 milliamps.21. Apparatus as claimed in claim 3 wherein the sensor is permanentlylocated relative to the DC system.
 22. A method as claimed in claim 12including permanently locating the sensor relative to the DC system. 23.Apparatus for the detection of low level ground leakage current in anormally ungrounded DC system in a multifeeder distribution systemhaving a relatively high capacitive reactance and being associated witha relatively strong electromagnetic and electrostatic field, said systemincluding a DC power supply, conductors from the supply for supplyingpower to a load connected in the DC system comprising a resistor elementfor connection across the DC power supply, a tapping point to theresistor element, a connector between the tapping point and a groundpoint such that a ground leakage current in the system completes aground circuit, a normally closed relay between the tapping point andground, a pulser for periodically opening and closing said relay whilethe ungrounded DC system remains substantially closed and operational,the opening and closing of said relay interrupting said ground circuiteffectively thereby to generate a DC pulse interrupted ground faultsignal being substantially a square wave, and a magnetic sensor forlocation relative to the DC system for sensing said magnetic fieldchanges and thereby said DC pulse interrupted ground fault signal andthereby providing for detecting the location of the low level groundleakage current in the relatively high capacitive reactance system, saidDC system remaining substantially closed and operational during theground leakage current detection, the pulse interruption rate beingbetween about 6 to about 11 cycles per second.
 24. A method of detectinglow level ground leakage current in a normally ungrounded DC system in amultifeeder distribution system having a relatively high capacitivereactance and being associated with a relatiely strong electromagneticand electrostatic field, said system including a DC power supply,conductors from the supply for supplying power to a load connected inthe DC system comprising the steps of connecting a resistor across theDC power supply, connecting an indicator between a tapping pointrelative to the resistor and a ground point such that a ground leakagecurrent in the system closes a ground circuit to activate the indicatormeans, periodically interrupting said circuit by opening and closing theground circuit effectively to generate a DC pulse interrupted groundfault signal being substantially a square wave to produce proportionalperiodic changes in a magnetic field associated with the system whilethe ungrounded DC system remains substantially operational, andmagnetically sensing in a location relative the DC system said magneticfield changes related to said pulse interrupted ground fault signalwhereby said magnetic field changes provide the necessary signal for lowlevel ground fault current detection, the pulse interruption rate beingbetween about 6 to about 11 cycles per second.